1. Field of the Invention
The present invention relates to disk drives and to the pattern of the servo burst fields written to disks used in such disk drives.
2. Description of the Prior Art and Related Information
In a conventional disk drive, each recording surface of each magnetic disk contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information is provided on this disk or another disk to determine the position of the head. The most popular form of servo is called “embedded servo” wherein the servo information is written on this disk in a plurality of servo sectors that are angularly spaced from one another and interspersed between data sectors around the track. Each servo sector generally comprises a track identification (ID) field and a group of servo bursts that the servo control system samples to align the transducer head with or relative to a particular data track. Each servo burst is conventionally formed from a series of magnetic transitions defined by an alternating pattern of magnetic domains.
The servo control system moves the transducer toward a desired track during a coarse “seek” mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a fine “track following” mode. The transducer generally reads the servo bursts to produce a Position Error Signal (PES) that is 0 when the transducer is at a particular radial position. The position where the PES=0 may or may not be at the data track center, depending on the magnetic characteristics of the transducer, the arrangement of the servo bursts, and the formula used to calculate the PES.
Conventionally, the servo burst written by the servo track writer include groups of four discrete servo bursts that are identified as A, B, C and D and that are disposed at predetermined positions relative to a track centerline. The A and B bursts may be thought of as in-phase and the C and D bursts are placed in “quadrature” with the A and B bursts, in that the edges of the C and D bursts may be aligned with the centers of the A and B bursts or may span a portion of a radial extent of the A and B bursts. With four bursts A, B, C, D positioned in quadrature, there are two burst pair centerlines (also called Track Center or TC herein) per data track pitch, i.e. one burst pair centerline every 50% of a data track pitch. The read/write transducer, therefore, will always pass over an A/B pair or a C/D pair of servo bursts because it is always within 25% of a data track pitch from an A/B or C/D burst pair centerline.
However, competitive pressures are such that disk drive designers continually seek to improve yield and capacity. The servo information conveyed by the servo bursts, although essential to the operation of the drive, is generally considered to be overhead. The area of the disk recording surface that is occupied by servo bursts cannot be used to record user data. Some of this recording surface real estate conventionally occupied by the four embedded A, B, C and D servo burst fields would be better utilized for recording user data. Improving the servo burst format by reducing the amount of the area of the disk recording surface that is occupied by the servo bursts would, in turn, correspondingly improve both yield and efficiency.